This invention relates to an improved hot-melt-coatable pressure-sensitive adhesive for use in adhering to low surface energy substrates and nonwovens. More specifically, the invention relates to a pressure-sensitive adhesive composition which provides superior adhesion characteristics when used against a polyethylene film or a nonwoven such as are used in typical diaper constructions.
U.S. Pat. No. 3,787,531 (Dahlquist) relates to a tacky pressure-sensitive adhesive described as having excellent shear strength and creep resistance at elevated temperatures. Dahlquist teaches that the use of tapered diblock elastomers produce adhesives that are useless for commercial pressure-sensitive adhesive tapes due to their tendencies to fail cohesively, specifically mentioning styrene-butadiene copolymers such as Solprene.TM. 1205, a tapered diblock copolymer. As a method of improving adhesive performance, Dahlquist proposes the use of pure diblock copolymers where there is no intermingling of monomers from one block in the block of the other, such as in a tapered block copolymer. The exemplified adhesives are comprised predominately of an A-B diblock copolymers of polyisoprene or polybutadiene elastomeric blocks tackified with solid tackifying resins, such as Piccolyte S-115 for the polyisoprene-based diblock adhesives and an alpha polyterpene tackifier for the polybutadiene-based diblock adhesives. The polymers obtained are described as having excellent adhesive properties, particularly high shear strength without the necessity of chemical cross-linking, however, these properties were determined against a conventional stainless steel substrate.
Other patents dealing with pressure-sensitive adhesives using A-B diblock copolymers with butadiene elastomeric blocks, particularly Solprene.TM. 1205, include U.S. Pat. No. 3,519,585 which describes an adhesive composition comprising 50-95 parts solid wood rosin, 50-95 parts of a styrene-butadiene diblock copolymer with correspondingly 10-50 parts of a styrene-butadiene triblock copolymer. When triblock was used alone, adhesives allegedly demonstrate poor tack strength, but had good shear strength. U.S. Pat. No. 3,592,710 describes an extrudable hot-melt adhesive composition comprised of an admixture of Solprene.TM. 1205, Kraton.TM. 1101 and a solid resin Pentalyn.TM. H. The exemplified adhesive comprises 70 parts Solprene.TM. 1205, 30 parks Kraton.TM. 1101 and 85 parts Pentalyn.TM. H. The adhesive allegedly displays good adhesion properties, while being extrudable. U.K. Patent No. 1,342,528 describes an adhesive composition based on Solprene.TM. 1205 and a triblock copolymer, cariflex.TM. 1101, both styrene-butadiene block copolymers. The Solprene.TM. 1205 is allegedly used in amounts up to 45 parts of the elastomer phase, the elastomer phase being tackified with a solid tackifier and an oil or liquid tackifier. It is alleged that the liquid tackifying agent surprisingly improves the surface tack or wet-grab properties of the adhesive without compromising shear resistance. The application only contemplates the use of the Solprene.TM. 1205 diblock copolymer in amounts up to 45 parts per 100 parts of the elastomeric phase, with Example 4 indicating that, at this level, tack significantly decreases.
U.S. Pat. No. 4,096,203 describes a method to obtain high tack adhesives which fail cohesively by varying the coupling efficiency of the elastomer. The elastomer comprises up to 80% A-B diblock copolymer.
The examples demonstrate coupling efficiency, at the low end, of 0 and 25% of styrene-isoprene/styrene-isoprene-styrene block copolymer systems tackified with C.sub.5 aliphatic hydrocarbon resin, alone and with an oil. Both systems failed cohesively, however the 0% coupled (100% diblock) system had significantly lower peel values and holding power.
U.S. Pat. No. 4,104,327 describes an adhesive prepared with pure triblock styrene-butadiene copolymer tackified with a solid tackifying resin and preferably an oil. It is alleged that the specific tackifying resin employed provides pressure-sensitive adhesives with superior properties versus compositions tackified with conventional terpene resins or aliphatic hydrocarbon resins, which are stated as having inferior tack, adhesive strength and cohesive force.
U.S. Pat. No. 4,944,993 discloses a hot-melt adhesive based on a radial styrene-butadiene copolymer with a solid tackifying resin and, optionally, an oil. The adhesive described is allegedly useful as a hot-melt construction adhesive, not a pressure-sensitive adhesive, for disposable articles such as sanitary napkins, hospital gowns, hospital bed pads, and the like. Specifically, it is stated that the adhesives show superior abilities to be used in spray fiberization techniques used in the construction of such disposable products. None of the above patents address the difficulties in adhering to low surface energy substrates or nonwovens, which problems are addressed by the present invention.